1. Field of the Invention
This invention relates to electro-optical devices, primarily for use in optical communications, that are capable of modulating the amplitude or phase of an optical output in response to an electrical data or control signal, or of switching it (for example, switching at a high speed, as in the context of a time-division demultiplexer).
2. Technical Background
“External” modulators reliant on the electro-optical effect, typically in lithium niobate, some semiconductors (e.g. GaAs and InP), “poled” polymers or “poled” glasses, are used to modulate light obtained from a laser. In this way, efficient data encoding can be achieved, even at high speeds. In principle, an electric field, typically with a frequency in the microwave region, is applied to the electro-active material and has the effect of changing its refractive index, whereby the speed at which light passes through it is changed and a phase change consequently induced in the light; usually, but not necessarily, this phase change is converted into a change in amplitude by an interferometric technique. A complication to this simple principle arises because a substantial optical path length is needed to achieve sufficient modulation depth, and the phase velocities of optical and microwave signals in electro-active materials are substantially different: for example, in lithium niobate, which is the most frequently used electro-active material, the ratio of the two velocities is around 2:1. The result of this is that “walk-off” would occur between the electric and optical signals within the active zone of the device, that is they would move progressively out of phase if suitable measures were not taken. The direction of modulation would reverse as they moved into antiphase and insufficient or even no resultant modulation would be obtained. Since the higher the microwave frequency, the greater the degree of mismatch, phase velocity mismatch leads to severe bandwidth limitations.
Adequate phase velocity matching can be achieved by appropriate design of the transverse geometry of the modulator, for example by the use of thick electrodes and/or of buffer layers to provide for partial propagation of the microwave mode in a medium of low dielectric constant.
Other techniques that are used to obtain phase matching are first the use of electrodes designed to delay the electrical signal in the waveguide by half a wavelength at the places where phase reversal would otherwise occur, and second the use of a body of electro-optical material that is periodically “poled” such that the direction of its electro-optic effect is reversed at those points. These techniques are primarily suitable for modulators for high-frequency narrow-band operation; they can be extended to low-frequency pass-band or large-bandwidth applications, for example by using structures with multiple periodicities or aperiodic structures, but always at the expense of significantly reduced modulation efficiency.
Another complicating factor is that microwave losses within the device are substantial and frequency-dependent (losses vary approximately in proportion to the square root of the frequency), and the combined result of these effects is that an increase in the length of the active region, which is desirable to reduce the voltage needed to obtain a phase difference of half a wavelength (or π) between the “on” and “off” conditions (sometimes hereinafter called the “switching voltage”), results in a substantial loss of bandwidth through the degradation of high-frequency signals.